Chapter 15 Power Supplies (oltage Regulators)
Power Supply Diagram 2
Filter Circuits The output from the rectifier section is a pulsating DC. The filter circuit reduces the peak-to-peak pulses to a small ripple voltage. 3
Ripple Factor After the filter circuit a small amount of AC is still remaining. The amount of ripple voltage can be rated in terms of ripple factor (r). %r ripple voltage (rms) dc voltage r(rms dc ) 100 4
Rectifier Ripple Factor Half-Wave Full-Wave DC output: DC output: dc 0.318 m dc 0.636 m AC ripple output: AC ripple output: r(rms) 0.385 m r(rms) 0.308 m Ripple factor: Ripple factor: %r r(rms) dc 0.385 0.318 100 m m 100 121% %r r(rms) dc 0.308 0.636 m m 100 100 48% 5
Types of Filter Circuits Capacitor Filter RC Filter 6
Capacitor Filter Ripple voltage 4 I 3fC 2.4I C dc dc r(rms) 2.4 R The larger the capacitor the smaller the ripple voltage. L dc C DC output dc m I dc 4fC m 4.17I C dc Ripple factor %r r(rms) dc 100 2.4I C dc dc 100 2.4 RLC 100 7
Diode Ratings with Capacitor Filter The size of the capacitor increases the current drawn through the diodes the larger the capacitance, the greater the amount of current. Peak Current vs. Capacitance: C I t where C capacitance change in capacitor voltage during charge/discharge t the charge/discharge time 8
RC Filter Circuit Adding an RC section further reduces the ripple voltage and decrease the surge current through the diodes. r(rms) X C R r(rms) % r(rms) ripple voltage after the RC filter r(rms) ripple voltage before the RC filter R resistor in the added RC filter X C reactance of the capacitor in the added RC filter NL FL 100% R FL NL no-load voltage FL full-load voltage FL 9
oltage Regulation Circuits There are two common types of circuitry for voltage regulation: Discrete Transistors IC s 10
Discrete-Transistor Regulators Series voltage regulator Current-limiting circuit Shunt voltage regulator 11
Series oltage Regulator Circuit The series element controls the amount of the input voltage that gets to the output. If the output voltage increases (or decreases), the comparator circuit provides a control signal to cause the series control element to decrease (or increase) the amount of the output voltage. 12
Series oltage Regulator Circuit R 1 and R 2 act as the sampling circuit Zener provides the reference voltage Q 2 controls the base current to Q 1 Q 1 maintains the constant output voltage When the output increases: When the output decreases: 1. The voltage at 2 and BE of Q 2 increases 2. The conduction of Q 2 increases 3. The conduction of Q 1 decreases 4. The output voltage decreases 1. The voltage at 2 and BE of Q 2 decreases 2. The conduction of Q 2 decreases 3. The conduction of Q 1 increases 4. The output voltage increases 13
Series oltage Regulator Circuit The op-amp compares the Zener diode voltage with the output voltage (at R 1 and R 2 ) and controls the conduction of Q 1. 14
Current-Limiting Circuit When I L increases: The voltage across R SC increases The increasing voltage across R SC drives Q 2 on Conduction of Q 2 reduces current for Q 1 and the load 15
Shunt oltage Regulator Circuit The shunt voltage regulator shunts current away from the load. The load voltage is sampled and fed back to a comparator circuit. If the load voltage is too high, control circuitry shunts more current away from the load. 16
Shunt oltage Regulator Circuit When the output voltage increases: The Zener current increases The conduction of Q 2 increases The voltage drop at R s increases The output voltage decreases When the output voltage decreases: The Zener current decreases The conduction of Q 2 decreases The voltage drop at R s decreases The output voltage increases 17
Shunt oltage Regulator Circuit 18
IC oltage Regulators Regulator ICs contain: Comparator circuit Reference voltage Control circuitry Overload protection Types of three-terminal IC voltage regulators Fixed positive voltage regulator Fixed negative voltage regulator Adjustable voltage regulator 19
Three-Terminal Terminal oltage Regulators The specifications for this IC indicate: The range of input voltages that can be regulated for a specific range of output voltage and load current Load regulation variation in output voltage with variations in load current Line regulation variation o in output voltage with variations at in input voltage 20
Fixed Positive oltage Regulator These ICs provide a fixed positive output voltage. 21
Fixed Negative oltage Regulator These ICs output a fixed negative output voltage. 22
Adjustable oltage Regulator These regulators have adjustable output voltages. The output voltage is commonly selected using a potentiometer. 23
Practical Power Supplies DC supply (linear power supplies) Chopper supply (switching power supplies) T horizontal high voltage supply Battery chargers 24